The invention relates to a method of generating images of layers of an object. The object is irradiated from a plurality of radiation source positions in order to generate separate single-shadow images. For each single-shadow image, measurement values are recorded, depending on the absorption at each image point. For each layer image, an image value is recorded for each layer-image point. The image value at each image point is derived from the recorded measurement values of the image points of the single-shadow images which are geometrically associated with the layer-image point.
An image point (or shadow-image point) is to be understood to mean herein a single-shadow image zone which has finite dimensions and which is preferably square. A layer-image point is to be understood to mean a corresponding zone in the layer image.
A method of this kind is described in U.S. Pat. No. 3,499,146. In this method the single-shadow images are generated by an X-ray source which is successively moved to different positions. The radiation pattern is recorded by an image converter, for example an image intensifier whose exit screen image is scanned by a television camera.
The video signal generated by the television camera represents the successive measurement values associated with the individual image points of the single-shadow image. The video signal can be stored in a digital or analog storage device.
The image value associated with a layer image point is derived from the shadow-image points which are geometrically associated with the layer-image point (i.e. those images points which are situated on the connecting line between the relevant layer-image point and the various radiation source positions during the exposure) by adding these measurement values. Additional steps may be taken to exclude values which deviate substantially from the other measurement values.
Due to the described superposition of the measurement values associated with the corresponding shadow-image points of the single-shadow images, details which are situated outside the imaged layer do not appear as strongly in the image of a layer image point than if they were situated in this imaged layer. Such points are thus "blurred" as in other tomography methods.
For objects which have comparatively spatially uniform absorption, suitable results are obtained by this method. However, for imaging objects having very inhomogeneous spatial absorption, for example coronary arteries filled with a contrast medium, the highly absorbent details situated outside the imaged layer are imaged several times in the layer. Such details appear more intense as the number of radiation source positions is decreased or the number of single-shadow images is decreased. These artifacts substantially influence the evaluation of such layer images.